Automotive Adhesives - From Design To Repair

Bruce Gascoigne
Commercial Development Manager
Lord Chemical Products Pty. Ltd.
Tullamarine, Australia

The unfamiliar and seemingly unpredictable behavior of adhesives has kept many of them out of automotive design, manufacturing, and repair. It's far easier for engineers to design joints using welds or mechanical fasteners that offer more exact, more predictable, and better understood performance. However, new vehicle designs using lighter gauge metals, coated steels, plastics, and nonferrous metals, are creating a need for new assembly methods. New aerodynamic structures have, in some cases, made it hard to use traditional spot-welding techniques. As a result, more and more adhesives are being recruited to do the joining.

Until recently, the needs of the repair industry seemed to be an afterthought for automotive companies. They provided minimal information to the aftermarket on methods of repairing new vehicles. Part of the problem is that most adhesives used by OEMs have no place in the body shop. For instance, structural, or load-bearing, adhesives are usually applied at the body-in-white stage when metal panels are fresh from the presses and covered in forming oil, and plastic panels are still covered in mold-release chemicals. To cure, they require temperatures as high as 130°C. Not only do aftermarket paint-bake ovens fall short of that mark, few car interiors would survive such high-temperature bake cycles.

For OEM designers, structural adhesives must be easy to apply and bond to surfaces despite the oil or mold release agents and withstand immersion into prewash detergents, phosphate, and e-coat baths. They should be able to cure and expand in the paint ovens at 150 to 204°C for 20 min. And, they must resist overbaking and overcuring if left in the ovens too long or put through repeated bake cycling.

With all these restrictions in mind, OEMs are investigating and approving new adhesive/welding approaches to repairing vehicles. Weld-bonding, for example, combines resistance spot welding with a structural adhesive. Lightweight steel cars made with tailored blanks and bake-hardenable steels, as well as aluminum, are using this joining technology as well as adhesive bonding, laser welding, and mechanical fastening. At Lotus, for example, engineers use a combination of mechanical fasteners and adhesives to assemble extruded aluminum frames.

WHY ADHESIVES?
Lighter structures mandate stronger parts with better load distribution. And consumers demand better handling and crashworthiness. In the past, one solution was to throw in a little more metal. Today, fuel economy standards demand a more inventive approach. Enter adhesives.

According to the Freedonia Group, a Cleveland-based research firm, the U.S. automotive industry's demand for adhesives and sealants is expected to grow 1.3% per year and reach 1.8 billion lb in 2003. Demand stems mostly from increased use of plastics but also because adhesives can strengthen structures, seal interiors, improve heating and air conditioning, and dampen vibration. They are also easy to use and cost effective. But engineers should know some basics of adhesives before using them in automotive designs, especially if they are concerned with aftermarket repairs.

DESIGNING FOR REPAIR
When qualifying body-shop-capable adhesives and sealants for repairing vehicles, automotive engineers should consider several factors. For example, fixed-glass is almost universally made as part of the vehicle's structure and is attached with polyurethane (PU) adhesives. Only special grades of PU can be used when replacing fixed-glass.

• Two-piece plastic panels and components are typically bonded using structural adhesives, whether the plastics are thermosetting (won't soften with heat), reinforced, or thermoplastic (will soften with heat). Typically, plastics are bonded with specially formulated PU or epoxy-type adhesives.
• Sheet-molding compound, the largest-volume plastic used in vehicles, contains large amounts of mold release. Only specially formulated adhesives can bond through the mold release.
• Fast-cure PU, acrylic, cyanoacrylate, and PSA tape adhesives are generally used to repair internal and external trim components. Though less critical in terms of structure, these adhesives are important to overall repairability. Simple primer systems let PU, acrylic, or PSA tapes be used in bonding and repair.
• Chemical type is a critical factor when selecting a repair adhesive. The adhesive's chemistry limits a product's use in some areas. For instance, certain chemistries are bound in specific adhesion properties, while others are limited by cure speed, flexibility, impact resistance, and sandability.

To assist OEMs and body repair technicians, some adhesive companies are generating data that can be used for predicting crash behavior. Combinations of rear impacts, rollovers, and frontal-offset impacts are done to see how adhesives perform under actual crash conditions and compared to both traditional weld repairs and cars fresh from the factory. Some tests show adhesive repairs on metal panels survive as well as traditional repairs and OEM assembly.

Thus far, only secondary panel repairs are considered practical for adhesives. When used on primary structures, adhesives can make the part too strong, which could affect the vehicle's crumple zones. However, as OEMs use more adhesives, vehicles should eventually be designed so that adhesives can be used to repair primary and secondary structures.